Compound of formula (I) (=HyDHC) is an important intermediate in the synthesis of 25-hydroxy-vitamin D3 (=HyD) (=Calcidiol).

Compound of formula (I) can be synthesized in a multistep synthesis from desmosterol or from the corresponding Cholesta-5,7,24-trien-3β-ol using a Diels-Alder adduct as disclosed in WO 93/21204 A1.
It has been observed that compound of formula (I) comprises relative high amounts of the impurity of formula (II) (=3β-Cholesta-5,7,22-triene-3,25-diol)

As this compound of formula (II) differs only by a C═C double bond (at carbon position 22) from compound of formula (I) the two compounds have very similar physical properties.
As compound (I) is transformed to 25-hydroxy-vitamin D3 (=HyD) using a photochemical reaction step, also the impurity, i.e. compound of formula (II), is transformed analogously to an impurity in the targeted end product (HyD).
Therefore, it is desired to avoid or strongly reduce such impurities in the end product.